1. Field of the Invention
The present invention relates to a noninvasive measurement of a concentration of a blood component. More particularly, the present invention relates to a method and apparatus for noninvasively measuring a concentration of a blood component using a differential absorption spectrum corresponding to a variation of amounts of blood and interstitial fluid in a blood vessel, the variation being generated by varying a thickness of a particular soft tissue of a subject.
2. Description of the Related Art
With overall improvements in quality of life and living conditions, interest in personal health has increased. As a result, a wide array of home medical equipment that allows people to easily monitor their personal health has been researched and developed. In a normal human body, bodily fluid is organically circulated and adjusted so that an amount of bodily fluid is maintained within a predetermined range. Bodily fluids include blood, urine, interstitial fluid, sweat, and saliva. In particular, concentrations of blood and urine (glucose and protein) are essential parameters in determining a person's state of health. In addition, concentrations of blood components, such as glucose, hemoglobin, bilirubin, cholesterol, albumin, creatinine, protein, and urea, play an important role in assessing a person's state of health.
When a human body is infected with a disease, a composition or amount of a component of a bodily fluid changes, which may result in death. For example, a normal person's blood glucose concentration is about 80 mg/dl before meal and about 120 mg/dl after meal. In order to maintain such a normal glucose concentration, a human pancreas secretes an appropriate amount of insulin before or after the meal so that glucose can be absorbed into the liver and skeletal muscle cells. However, when the pancreas does not secrete an appropriate amount of insulin to maintain a normal blood glucose concentration due to a disease or other causes, an excessive amount of glucose exists in the blood, which causes a disease of the heart or the liver, arteriosclerosis, hypertension, cataract, retinal bleeding, nerve damage, hearing loss, or visual disturbance, all of which may cause serious problems including death. Accordingly, a technique of measuring a change in a bodily fluid of a human body is considered very important.
Methods of measuring a concentration of a component of a bodily fluid include invasive methods of directly collecting a sample of a target matter and performing measurement on the collected sample of the target matter and noninvasive methods of performing measurement without directly collecting a target matter. Since invasive methods have many problems, techniques of easily analyzing components of a bodily fluid using a noninvasive method have been continuously researched and developed. Conventionally, when measuring a component of a bodily fluid, for example, blood glucose, blood is extracted, reacted with a reagent, and then analyzed by using a clinical analysis system or quantifying a change in color of a test strip. When such a blood glucose test is performed every day, a patient suffers pain resulting from the direct blood collection and is susceptible to infection. Moreover, since it is difficult to continuously monitor the blood glucose level, it is difficult to properly treat a patient in an emergency situation. In addition, use of disposable strips and reagents may be a financial burden on the patient. Furthermore, these disposable strips and reagents cause environmental contamination, and as such, require special treatment. Accordingly, development of a technique of measuring a blood glucose concentration without extracting blood is desired for monitoring and adjusting a blood glucose level of a diabetic or diagnosing a person's state of health. Many methods of noninvasively measuring blood glucose have been researched, but instruments using these methods have not been commercialized.
In most conventional, spectroscopic methods for measuring a concentration of a blood component in a human body, light within a visible ray and near infrared ray (NIR) wavelength range is radiated onto a part of the body. Then, light reflected from or transmitted through the body is detected. In such spectroscopic methods, a spectrum is usually measured to measure the concentration of a blood component. Here, a reference light source having a wavelength that best responds to a blood component to be measured and a bandwidth that effectively counterbalances an influence of an interference substance is required. In addition, since a concentration of a component to be measured may be very low in blood and a light diffusion effect is greater than a light absorption effect in living tissue and blood, a detected signal is very weak. Thus, a method of amplifying the signal is required. Moreover, since organic substances in the body flow continuously, a component concentration can be accurately measured only when the measurement is quickly performed. In addition, it must be noted that an average energy radiated onto a human body should not go beyond a limit that may damage the human body. In particular, in an NIR wavelength range of about 700 through 2500 nm, a glucose absorption band is widely distributed, and glucose absorption is small as compared to a large aqueous background spectrum. Resultantly, a signal to noise ratio (SNR) is small, which makes accurate measurement very difficult.